Back in the days that I worked for
Aviation Security Service, I was asked to be part of a
Drug Dog test (dogs get tested
every few months to continue in the service). The
test is also testing the
handler, who wasn't
privy to
what drug was being found, or
whom was holding it.
The test involved a whole bunch of volunteers to line up at the empty (because no international flights were due in for another half hour) immigration counters as if they were normal every-day passengers. The police had a cap of oil (capsule of marijuana oil) wrapped up in tinfoil, which was in turn wrapped up in two large plastic bags with rubber bands holding it all together. They put this down the sock of one of the volunteers, and everyone was asked to just ignore the dog as it came around.
The dog was led up and down one isle and then the next, but was extremely keen to head for the fourth where the drugs were. The handler kept the dog on track, sniffing everyone and their "luggage" (any hand-held items as we weren't real passengers) until they reached the fourth row. Then the dog bolted for the guy with the drugs, nearly tearing the arm off the handler. The dog jumped up once on the volunteer with the drugs, but was pulled back, and then sat beside him staring at him.
The handler pointed at the volunteer, looking at the testers, and said "This guy's got drugs". The testers went over and got the drugs out of the volunteer's sock, and said something about it being a low-odour sample too, and we all went our separate ways. All up, an extremely impressive display of the power of a dog's nose.
On another note, the bomb-detector dog was often sniffing around our offices as a bit of practice too. They had wrapped samples of gunpowder and other explosives that they hid in various locations (where no humans would accidentally find them) and get the dog to sniff them out.